Flow battery system

ABSTRACT

In accordance with embodiments of the present disclosure, a redox flow battery (RFB) may include a shell, an electrolyte storage tank assembly disposed in the shell, wherein at least a portion of the electrolyte storage tank assembly is supported by the shell, an electrochemical cell, and an electrolyte circulation system configured for fluid communication between the electrolyte storage tank assembly and the electrochemical cell. In some embodiments, at least a portion of the electrolyte storage tank assembly defines a tank assembly heat transfer system between an outer surface of the electrolyte storage tank assembly and an inner surface of the shell. In other embodiments, a pump assembly in the electrolyte circulation system is moveable between a first position and a second position. In other embodiments, a gas management system includes a first gas exchange device in fluid communication with the catholyte headspace and the anolyte.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/607,842, filed Dec. 19, 2017, the disclosure of which is expresslyincorporated by reference herein in its entirety.

BACKGROUND

Concerns over the environmental consequences of burning fossil fuelshave led to an increasing use of renewable energy generated from sourcessuch as solar and wind. The intermittent and varied nature of suchrenewable energy sources, however, has made it difficult to fullyintegrate these energy sources into existing electrical power grids anddistribution networks. A solution to this problem has been to employlarge-scale electrical energy storage (EES) systems. These systems arewidely considered to be an effective approach to improve thereliability, power quality, and economy of renewable energy derived fromsolar or wind sources.

In addition to facilitating the integration of renewable wind and solarenergy, large scale EES systems also may have the potential to provideadditional value to electrical grid management, for example: resourceand market services at the bulk power system level, such as frequencyregulation, spinning reserves, fast ramping capacity, black startcapacity, and alternatives for fossil fuel peaking systems; transmissionand delivery support by increasing capability of existing assets anddeferring grid upgrade investments; micro-grid support; and peak shavingand power shifting.

Among the most promising large-scale EES technologies are redox flowbatteries (RFBs). RFBs are special electrochemical systems that canrepeatedly store and convert megawatt-hours (MWhs) of electrical energyto chemical energy and chemical energy back to electrical energy whenneeded. RFBs are well-suited for energy storage because of their abilityto tolerate fluctuating power supplies, bear repetitive charge/dischargecycles at maximum rates, initiate charge/discharge cycling at any stateof charge, design energy storage capacity and power for a given systemindependently, deliver long cycle life, and operate safely without firehazards inherent in some other designs.

In simplified terms, an RFB electrochemical cell is a device capable ofeither deriving electrical energy from chemical reactions, orfacilitating chemical reactions through the introduction of electricalenergy. In general, an electrochemical cell includes two half-cells,each having an electrolyte. The two half-cells may use the sameelectrolyte, or they may use different electrolytes. With theintroduction of electrical energy, species from one half-cell loseelectrons (oxidation) to their electrode while species from the otherhalf-cell gain electrons (reduction) from their electrode.

Multiple RFB electrochemical cells electrically connected together inseries within a common housing are generally referred to as anelectrochemical “stack”. Multiple stacks electrically connected togetherare generally referred to as a “string”. Multiple stings electricallyconnected together are generally referred to as a “site”.

A common RFB electrochemical cell configuration includes two opposingelectrodes separated by an ion exchange membrane or other separator, andtwo circulating electrolyte solutions, referred to as the “anolyte” and“catholyte”. The energy conversion between electrical energy andchemical potential occurs instantly at the electrodes when the liquidelectrolyte begins to flow through the cells.

To meet industrial demands for efficient, flexible, rugged, compact, andreliable large-scale ESS systems with rapid, scalable, and low-costdeployment, there is a need for improved RFB systems.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

In accordance with one embodiment of the present disclosure, a redoxflow battery (RFB) is provided. The redox flow battery (RFB) includes: ashell; an electrolyte storage tank assembly disposed in the shell,wherein at least a portion of the electrolyte storage tank assembly issupported by the shell and wherein at least a portion of the electrolytestorage tank assembly defines a tank assembly heat transfer systembetween an outer surface of the electrolyte storage tank assembly and aninner surface of the shell; an electrochemical cell; and an electrolytecirculation system configured for fluid communication between theelectrolyte storage tank assembly and the electrochemical cell.

In accordance with another embodiment of the present disclosure, a tankand shell secondary containment system is provided. The system includes:a shell; and a tank disposed within the shell, wherein at least aportion of the tank is supported by the shell and wherein at least aportion of the tank defines a heat transfer system between an outersurface of the tank and an inner surface of the shell, wherein the heattransfer system includes a plurality of air flow channels and an aircirculation device.

In accordance with another embodiment of the present disclosure, amethod of heat transfer for a redox flow battery (RFB) is provided. Themethod includes: operating a redox flow battery having a shell, anelectrolyte storage tank assembly disposed in the shell, wherein atleast a portion of the electrolyte storage tank assembly is supported bythe shell and wherein at least a portion of electrolyte storage tankassembly defines a tank assembly heat transfer system between an outersurface of the electrolyte storage tank assembly and an inner surface ofthe shell, an electrochemical cell, and an electrolyte circulationsystem configured for fluid communication between the electrolytestorage tank and the electrochemical cell; and circulating air throughthe tank assembly heat transfer system between an outer surface of theelectrolyte storage tank assembly and the inner surface of the shell.

In any of the embodiments described herein, the heat transfer system mayinclude a plurality of air flow channels and an air circulation device.

In any of the embodiments described herein, the tank assembly heattransfer system may include a plurality of tank abutments and aplurality of tank channels, with two tank abutments adjacent eachchannel.

In any of the embodiments described herein, the electrolyte storage tankassembly may include one or more tank walls having a wall thickness.

In any of the embodiments described herein, the one or more tank wallsmay have substantially constant wall thickness.

In any of the embodiments described herein, the one or more tank wallsmay have variable wall thickness.

In any of the embodiments described herein, the redox flow battery maybe a vanadium redox flow battery.

In any of the embodiments described herein, the electrolyte storage tankassembly may include a catholyte tank and an anolyte tank.

In any of the embodiments described herein, the catholyte tank and theanolyte tank may be in a side-by-side configuration in the shell.

In any of the embodiments described herein, each of the catholyte tankand the anolyte tank may define a portion of the tank assembly heattransfer system between an outer surface of each tank and an innersurface of the shell.

In any of the embodiments described herein, the anolyte tank has avolume and wherein the catholyte tank has a volume, and the ratio of thevolume of the anolyte tank to the volume of the catholyte tank may be inthe range of 1.05:1 to about 1.5:1.

In any of the embodiments described herein, the catholyte tank and theanolyte tank may have substantially the same footprint in contact with abottom surface of the shell.

In any of the embodiments described herein, the catholyte tank and theanolyte tank may have substantially the same liquid level.

In any of the embodiments described herein, the catholyte tank mayinclude a stepped shelf to reduce the volume of the catholyte tankcompared to the anolyte tank.

In accordance with another embodiment of the present disclosure, a redoxflow battery (RFB) is provided. The redox flow battery (RFB) includes: ashell having a shell height; an electrolyte storage tank assemblydisposed in the shell, wherein at least a portion of the electrolytestorage tank assembly is supported by the shell, the electrolyte storagetank assembly having an electrolyte liquid height, wherein theelectrolyte liquid height is at or below the shell height; anelectrochemical cell; and an electrolyte circulation system configuredfor fluid communication between the electrolyte storage tank assemblyand the electrochemical cell, wherein the electrolyte circulation systemincludes a pump assembly, wherein the pump assembly is moveable betweena first position in the shell and below the electrolyte liquid heightduring operation of the pump assembly and a second position and abovethe electrolyte liquid height when the pump assembly is not operating.

In accordance with another embodiment of the present disclosure, a tanksystem configured for holding a liquid is provided. The system includes:a shell having a shell height; a tank disposed within the shell, whereinat least a portion of the tank is supported by the shell and having aliquid height, wherein the liquid height is at or below the shellheight; and an liquid circulation system configured including a pumpassembly, wherein the pump assembly is moveable between a first positionin the shell and below the liquid height during operation of the pumpassembly and a second position above the electrolyte liquid height whenthe pump assembly is not operating.

In accordance with another embodiment of the present disclosure, amethod of servicing a pump in a redox flow battery (RFB) is provided.The method includes: operating a redox flow battery having a shellhaving a shell height, an electrolyte storage tank assembly disposed inthe shell, wherein at least a portion of the electrolyte storage tankassembly is supported by the shell, the electrolyte storage tankassembly having an electrolyte liquid height, wherein the electrolyteliquid height is at or below the shell height, an electrochemical cell,and an electrolyte circulation system configured for fluid communicationbetween the electrolyte storage tank assembly and the electrochemicalcell, wherein the electrolyte circulation system includes a pumpassembly, wherein the pump assembly is moveable between a first positionin the shell and below the electrolyte liquid height during operation ofthe pump assembly and a second position above the electrolyte liquidheight when the pump assembly is not operating; turning off the pumpassembly; and moving the pump assembly to the second position.

In any of the embodiments described herein, the electrolyte storage tankassembly may include a shelf located at a height above the bottom of theelectrolyte storage tank assembly and below the shell height defining aspace within the shell.

In any of the embodiments described herein, the pump assembly may belocated in the space within the shell.

In any of the embodiments described herein, the pump assembly may becoupled to first and second connections of the electrolyte circulationsystem when in the first position.

In any of the embodiments described herein, the pump assembly may beuncoupled from a first connection to the electrolyte circulation systemand may remain coupled to a second connection to the electrolytecirculation system when in the second position.

In any of the embodiments described herein, the pump assembly may berotatable between the first position and the second position whilecoupled to the second connection in the electrolyte circulation system.

In any of the embodiments described herein, the pump may include afilter and/or a union.

In any of the embodiments described herein, the redox flow battery maybe a vanadium redox flow battery.

In any of the embodiments described herein, the electrolyte storage tankassembly may include a catholyte tank and an anolyte tank.

In any of the embodiments described herein, the catholyte tank and theanolyte tank may be in a side-by-side configuration in the shell.

In any of the embodiments described herein, each of the catholyte tankand the anolyte tank may define a portion of the tank heat transfersystem between an outer surface of each tank and an inner surface of theshell.

In any of the embodiments described herein, the anolyte tank has avolume and wherein the catholyte tank has a volume, and the ratio of thevolume of the anolyte tank to the volume of the catholyte tank may be inthe range of 1.05:1 to about 1.5:1.

In any of the embodiments described herein, the catholyte tank and theanolyte tank may have substantially the same footprint in contact with abottom surface of the shell.

In any of the embodiments described herein, the catholyte tank and theanolyte tank may have substantially the same liquid level.

In any of the embodiments described herein, the catholyte tank mayinclude a stepped shelf to reduce the volume of the catholyte tankcompared to the anolyte tank.

In any of the embodiments described herein, the pump assembly mayinclude catholyte and anolyte pumps.

In any of the embodiments described herein, the pump assembly mayinclude catholyte and anolyte filters.

In any of the embodiments described herein, the pump assembly in asecond position may be above the electrolyte liquid height when the pumpassembly is not operating.

In accordance with another embodiment of the present disclosure, a redoxflow battery (RFB) is provided. The redox flow battery (RFB) includes:an anolyte storage tank configured for containing a quantity of anolyteand an anolyte headspace; a catholyte storage tank configured forcontaining a quantity of a catholyte and a catholyte headspace; and agas management system comprising a first gas exchange device having afirst end in fluid communication with the catholyte headspace and asecond end in fluid communication with anolyte in the anolyte storagetank.

In accordance with another embodiment of the present disclosure, amethod of operating a redox flow battery is provided. The methodincludes: operating an RFB having electrolyte storage tank assemblyincluding an anolyte storage tank configured for containing a quantityof anolyte and an anolyte headspace and a catholyte storage tankconfigured for containing a quantity of a catholyte and a catholyteheadspace, an electrochemical cell, and an electrolyte circulationsystem configured for fluid communication between the electrolytestorage tank assembly and the electrochemical cell; transferring gasfrom the catholyte headspace and depositing the gas the anolyte in theanolyte storage tank.

In any of the embodiments described herein, the first gas exchangedevice may include a gas treatment zone for treating evolving gas thatis evolving from the catholyte.

In any of the embodiments described herein, the evolving gas may includeoxygen, carbon dioxide, hydrogen, and chlorine, and any combinationthereof.

In any of the embodiments described herein, the redox flow battery maybe selected from the group consisting of a vanadium-sulfate redox flowbattery, a vanadium-chloride redox flow battery, a vanadium-mixedsulfate and chloride battery, a vanadium-iron redox flow battery, and aniron-chromium redox flow battery.

In any of the embodiments described herein, the redox flow battery maybe a vanadium redox flow battery.

In any of the embodiments described herein, the gas treatment zone mayinclude UV treatment.

In any of the embodiments described herein, chlorine and hydrogenevolving gases may be recombined to form hydrogen chloride.

In any of the embodiments described herein, the UV treatment may promotethe recombination of hydrogen and chlorine gas into hydrogen chloride.

In any of the embodiments described herein, the first gas exchangedevice may include a vacuum to draw gas from the catholyte headspace.

In any of the embodiments described herein, the first end of the firstgas exchange device may include a venturi.

In any of the embodiments described herein, the second end of the firstgas exchange device may be below the liquid level in the anolyte.

In any of the embodiments described herein, the gas treatment zone mayinclude a heat sensor.

In any of the embodiments described herein, the gas management systemmay further include a second gas exchange device for gas exchangebetween the catholyte headspace and the anolyte headspace.

In any of the embodiments described herein, the gas management systemmay further include a third gas exchange device configured to contain orrelease an evolving gas from either or both of the anolyte and catholytestorage tanks to an exterior battery environment when an interiorbattery pressure exceeds an exterior battery pressure by a predeterminedamount.

In any of the embodiments described herein, the third gas exchangedevice may be a liquid-filled U-shaped tube.

In any of the embodiments described herein, the third gas exchangedevice may include an arrangement of one or more of pressure-regulated,pressure relief, or check valves.

In any of the embodiments described herein, a method of operation mayfurther include treating the gas with treatment before depositing thegas in to a location at or below the liquid level of the anolyte in theanolyte storage tank.

In any of the embodiments described herein, the treatment may be UVtreatment.

In any of the embodiments described herein, the gas may be transferredto a location at or below the liquid level of the anolyte in the anolytestorage tank.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thepresent disclosure will become more readily appreciated as the samebecome better understood by reference to the following detaileddescription, when taken in conjunction with the accompanying drawings,wherein:

FIG. 1 is an isometric view of a redox flow battery in accordance withone embodiment of the present disclosure;

FIG. 2 is a partially exploded isometric view of the redox flow batteryof FIG. 1;

FIG. 3 is an isometric view of major internal components of the redoxflow battery of FIG. 1 including anolyte and catholyte tanks, anelectrochemical cell, a system for circulating electrolyte, and a gasmanagement system;

FIG. 4 is a schematic view of various components of a redox flow batteryin accordance with embodiments of the present disclosure;

FIG. 5 is a schematic view of power architecture for a redox flowbattery in accordance with one embodiment of the present disclosure;

FIG. 6 is a close-up, rear isometric view of various components of theredox flow battery of FIG. 1;

FIG. 7 is an isometric view of the tank assembly of the redox flowbattery of FIG. 1 showing exemplary air flow paths around the tank;

FIG. 8 is a bottom view of the tank assembly of the redox flow batteryof FIG. 1;

FIGS. 9A and 9B are cross-sectional views through exemplary tank wallsof redox flow batteries in accordance with embodiments of the presentdisclosure;

FIG. 10 is a cross-sectional view of major components of the redox flowbattery of FIG. 1 showing exemplary electrolyte travel paths;

FIG. 11 is an isometric view of a system for circulating electrolyte inthe redox flow battery of FIG. 1 showing exemplary electrolyte travelpaths;

FIGS. 12A and 12B are front views of a pumping system for the redox flowbattery of FIG. 1, showing different pump assembly configurations;

FIGS. 13A-13D are various views of exemplary conduit coupling for theredox flow battery of FIG. 1;

FIG. 14 is a schematic view of a redox flow battery system in accordancewith one embodiment of the present disclosure, showing exemplarychemical properties of the system; and

FIG. 15 is an isometric view of a system for gas management in the redoxflow battery of FIG. 1 showing exemplary gas management components.

DETAILED DESCRIPTION

Embodiments of the present disclosure are directed to redox flowbatteries (RFBs), systems and components thereof, stacks, strings, andsites, as well as methods of operating the same. Referring to FIGS. 1-4,a redox flow battery 20 in accordance with one embodiment of the presentdisclosure is provided. Multiple redox flow batteries may be configuredin a “string” of batteries, and multiple strings may be configured intoa “site” of batteries.

As with other battery systems, a redox flow battery 20 is configured tostore energy from an energy source and supply it when needed. Multipleredox flow batteries may be electrically connected in series or inparallel depending on the design of the system.

Redox Flow Battery

Referring to FIGS. 1-4, major components in an RFB 20 include a tankassembly 26 including anolyte and catholyte tanks 22 and 24, theelectrochemical cell 30, a system for circulating electrolyte 40, a gasmanagement system 94, and a shell 50 to house all of the components andprovide secondary liquid containment. In the illustrated embodiment, theRFB 20 includes one electrochemical cell 30. However, in otherembodiments of the present disclosure, and RFB 20 may include a stack ofmultiple electrochemical cells, for example, as described in U.S. Pat.No. 9,722,264, issued Aug. 1, 2017, the disclosure of which is expresslyincorporated by reference herein in its entirety.

Referring to FIG. 2, a control box 36 is coupled to the electrochemicalcell 30 via contact lines 51 and 52 to monitor and control the system(see also FIG. 6).

In the present disclosure, flow electrochemical energy systems aregenerally described in the context of an exemplary vanadium redox flowbattery (VRB), wherein a V³⁺/V²⁺ electrolyte serves as the negativeelectrolyte (“anolyte”) and a V⁵⁺/V⁴⁺ electrolyte serves as the positiveelectrolyte (“catholyte”). However, other redox chemistries arecontemplated and within the scope of the present disclosure, including,as non-limiting examples, V²⁺/V³⁺ vs. Br⁻/ClBr₂, Br₂/Br⁻ vs. S/S²⁻,Br⁻/Br₂ vs. Zn²⁺/Zn, Ce⁴⁺/Ce³⁺ vs. V²⁺/V³⁺, Fe³⁺/Fe²⁺ vs. Br₂/Br⁻,Mn²⁺/Mn³⁺ vs. Br₂/Br⁻, Fe³⁺/Fe²⁺ vs. Ti²⁺/Ti⁴⁺, etc.

As a non-limiting example, in a vanadium flow redox battery (VRB) priorto charging, the initial anolyte solution and catholyte solution eachinclude identical concentrations of V³⁺ and V⁴⁺. Upon charge, thevanadium ions in the anolyte solution are reduced to V²⁺/V³⁺ while thevanadium ions in the catholyte solution are oxidized to V⁴⁺/V⁵⁺.

In accordance with embodiments of the present disclosure, the anolyteand catholyte in a VRB may include vanadium ions and hydrochloric acidor a mix of sulfuric acid and hydrochloric acid. In accordance withother embodiments of the present disclosure, the anolyte and catholytein a VRB may include sulfate chemistry.

Referring to the schematic in FIG. 3A, general operation of the redoxflow battery system 20 of FIGS. 1 and 2 will be described. The redoxflow battery system 20 operates by circulating the anolyte and thecatholyte from their respective tanks 22 and 24 into the electrochemicalcell 30. The cell 30 operates to discharge or store energy as directedby power and control elements in electrical communication with theelectrochemical cell 30.

In one mode (sometimes referred to as the “charging” mode), power andcontrol elements connected to a power source operate to store electricalenergy as chemical potential in the anolyte and catholyte. The powersource can be any power source known to generate electrical power,including combustion power sources, as well as renewable power sources,such as wind, solar, and hydroelectric.

In a second (“discharge”) mode of operation, the redox flow batterysystem 20 is operated to transform chemical potential stored in theanolyte and catholyte into electrical energy that is then discharged ondemand by power and control elements that supply an electrical load.

Each electrochemical cell 30 in the system 20 includes a positiveelectrode, a negative electrode, at least one catholyte channel, atleast one anolyte channel, and an ion transfer membrane separating thecatholyte channel and the anolyte channel. The ion transfer membraneseparates the electrochemical cell into a positive side and a negativeside. Selected ions (e.g., H+) are allowed to transport across an iontransfer membrane as part of the electrochemical charge and dischargeprocess. The positive and negative electrodes are configured to causeelectrons to flow along an axis normal to the ion transfer membraneduring electrochemical cell charge and discharge (see, e.g., lines 51and 52 in FIGS. 2 and 4). As can be seen in FIG. 3A, fluid inlets 48 and44 and outlets 46 and 42 are configured to allow integration of theelectrochemical cell 30 into the redox flow battery system 20.

To obtain high voltage, high power systems, a plurality of singleelectrochemical cells may be assembled together in series to form astack of electrochemical cells (referred to herein as a “stack,” a “cellstack,” or an “electrochemical cell stack”. In the illustratedembodiment, the stack includes two half-stacks (or half cells) to form abattery system 20. Likewise, several cell stacks may then be furtherassembled together to form a battery system 20. A MW-level RFB systemgenerally has a plurality of cell stacks, for example, with each cellstack having a plurality of electrochemical cells. As described for anindividual electrochemical cell, the stack can be arranged with positiveand negative current collectors that cause electrons to flow through thecell stack generally along an axis normal to the ion transfer membranesand current collectors during electrochemical charge and discharge (see,e.g., lines 51 and 52 shown in FIG. 4 for a single stack battery andalso shown in FIG. 6).

The ion exchange membrane in each electrochemical cell preventscrossover of the active materials between the positive and negativeelectrolytes while supporting ion transport to complete the circuit. Ionexchange membrane material, in a non-limiting example, a perfluorinatedmembrane such as NAFION or GORE-SELECT, may be used in theelectrochemical cells.

Ion exchange through the membrane ideally prevents the transport ofactive materials between the anolyte and catholyte. However, dataobtained from operating vanadium redox batteries (VRBs) shows capacityfading over time when the system is operating without any capacityfading mitigation features as described herein. Such capacity fadingmay, at least in part, be attributed to some transport of vanadium ionsacross the membrane. Different vanadium cations in the system havedifferent concentration diffusion coefficients and electric-migrationcoefficients for crossing over through the membrane. These differencescontribute to an unbalanced vanadium transfer between anolyte andcatholyte after multiple cycles of operation, which may result in a lossof energy storage capacity.

Other negative effects caused by the transport of vanadium ions acrossthe membrane include precipitation, which may occur if the vanadium ionconcentration in the catholyte continues to increase as a result of thenet transfer of vanadium ions. Precipitate may form in the electrodestacks, which may result in degradation in the performance of the VRBsystem. As a non-limiting example, precipitation of V⁵⁺ as V₂O₅ canoccur in the catholyte (thereby decreasing the amount and/or theconcentration and amount of V⁵⁺ in the catholyte).

In addition to the transport of vanadium ions across the membrane andprecipitation, other electrochemical side reactions may contribute todecreased performance in VRB systems. These reactions must also beaddressed to maximize the capacity and service life of the system, whileminimizing cost and service requirements for the life of the battery.For example, under some operating conditions, side reactions may produceexcess hydrogen and chlorine gases in the headspaces of the anolyteand/or catholyte tanks. Other detrimental reactions may also occur whenelectrolyte is exposed to oxidizing agents such as oxygen. In oneexample, over time, the anolyte is susceptible to V²⁺ oxidation byatmospheric oxygen that is introduced into the tank during maintenance,installation, or other operations (thereby decreasing the amount and/orconcentration of V²⁺). As another example, hydrogen can be generated asa consequence of insufficient supply of reactants in the anolyte.

Described herein are systems and methods of operation designed formitigating the capacity decaying effects described above to improve RFBperformance on a battery, string, and site level. In general, thesefeatures can be described in terms of maintaining electrolyte stabilityby applying active and passive charge balancing, employing specificmethods for analysis and adjustment of electrolyte composition, andprocess gas management.

Power Architecture

Referring to FIG. 5, an exemplary schematic diagram of powerarchitecture for a redox flow battery 20 is provided in accordance withone embodiment of the present disclosure. The major components of thesystem in the power architecture include a power distribution unit 12,the battery 20, and contacts 51 and 52. Auxiliary loads powered by thepower distribution unit as shown in the diagram include electrolytepumps 120, cooling fan 32, and the BMS 14 (a battery management system).The auxiliary loads may be powered by external AC or the stack DC power.

The BMS is a controller which implements operational logic for the redoxflow battery. The BMS may provide an interface for higher level control,which may allow for operation and data acquisition. The BMS may be usedto connect or disconnect the battery, and when connected, to operate thebattery within safe parameters.

When the battery is connected, the series contactors are closed and theBMS controls the speed of the electrolyte pumps and cooling fan tooptimize the system efficiency. If the battery is part of a string,state of charge (SOC) matching may be performed.

When the battery is disconnected, the series contactors are open, andthe pumps and fans are disabled. However, if the battery is configuredfor black start (which is preserving power reserves to restart andrestarting by using stored power in the battery system), the batterywill enable the pumps periodically to maintain useable energy in thestack.

Battery Containment System, Electrolyte Tank Assembly, and GeneralArrangement

Referring now to FIGS. 1 and 2, each RFB 20 includes a shell 50 thathouses components of the system in a substantially closed manner.Referring to FIG. 3, these components generally include the tankassembly 26 including anolyte and catholyte tanks 22 and 24, theelectrochemical cell 30, a system for circulating electrolyte 40, and agas management system 94. The configuration of each of these componentswill now be described in more detail.

FIGS. 1 and 2 depict the shell 50 that houses, for example, thecomponents shown in FIG. 3. The container 50 can be configured in someembodiments to be an integrated structure that facilitates or providesone or more of the following characteristics: compact design, ease ofassembly, transportability, compact multiple-container arrangements andstructures, accessibility for maintenance, and secondary containment.

As will be described in more detail below, the tanks 22 and 24 of theRFB 20 are configured so as to be closely fitted within the compartmentor compartments, thereby maximizing the storage volume of electrolytewithin the shell 50 and providing structural support and non-permeablecontainment for the volume of electrolyte in the RFB 20, which isdirectly proportional to the energy storage of the battery 20.

The shell 50 is generally sized to fit within a standard door size forease of shipping and ease of installation. In some embodiments, theshell 50 has a standard dimensioning of 30″ in width by 57″ in depth and78″ in height. In some of these embodiments, the shell 50 can beadditionally configured to meet shipping container certificationstandards for registration and ease of transportation via rail, cargoship, or other possible shipping channels.

The shell 50 also includes various features to allow for the RFB 20 tobe easily placed in service and maintained on site. For example, aremovable lid 60 from the main compartment 62 of the shell 50, anelectrical box 36 located at the top of the RFB 20 for ease of access,and pumps and other serviceable components configured for accessibilityfor replacement or repair.

The shell 50 may be manufactured from any suitable materials, includingbut not limited to continuously welded or extruded metal (such as sheetsteel, stainless steel, titanium, or aluminum) or plastics. The shell 50may be coated to be chemical resistant. Although secondary containmentof the redox flow battery 20 and the electrolyte stored in the RFB 20 bythe shell 50 is generally desirable, secondary containment by a shell 50may not be needed for some battery deployments. In some embodiments, theshell 50 may be a single integrated component. Likewise, in somedeployments, the main compartment 62 of the shell 50 may be used withouta lid 60.

Electrolyte Tank and Assembly

Referring to FIGS. 7 and 8, the tank assembly 26 including the anolyteand catholyte tanks 22 and 24 provides primary containment structuresfor the anolyte and catholyte in the redox flow battery 20 in separateenclosures. FIG. 7 illustrates the anolyte and catholyte tanks 22 and 24positioned side-by-side (or front-and-back) in the main compartment 62.FIG. 8 shows a bottom view of the anolyte tank 22 and the catholyte tank24 positioned side-by-side (or front-and-back) in the main compartment62. In a side-by-side (or front-and-back) configuration, the tanks arealigned along one surface or one wall. In the illustrated embodimentshown in FIGS. 7 and 8, the representative bottoms of the anolyte andcatholyte tanks 22 and 24 are generally rectangular.

Referring to FIG. 7, the top portion of the catholyte tank 24 has ashoulder or stepped shelf section 90 located at the front upper cornerof the forward positioned tank (which is the catholyte tank 24 in theillustrated embodiment). The stepped shelfs section 90 is located at aheight above the bottom of the tank assembly 26 and below the maincompartment 62 of the shell 50 height defining a space within the shell50.

The space in the shell 50 (defined by the stepped shelf section 90)provides access for an electrolyte transfer conduit 92 (see FIG. 3) toprovide fluid communication between the anolyte tank 22 and thecatholyte tank 24 when the tanks are aligned side-by-side, as describedin greater detail below. The space created by the stepped shelf section90 also provides a location for the pump assembly 120 including anolyteand catholyte pumps 122 and 124 and filters 150 (see FIG. 3), asdescribed in greater detail below. The pump assembly 120 may alsoinclude optional unions. However, in accordance with other embodimentsof the present disclosure, one or both of the tanks 22 and 24 need notbe manufactured to include a stepped shelf section 90 or may includeanother configuration to optionally accommodate the electrolyte transferconduit 92 and/or the anolyte and catholyte pumps 122 and 124.

In some embodiments, anolyte tank 22 and/or catholyte tank 24 areconstructed from molded or fabricated plastic, fiberglass, or othermaterials or combinations of materials. In some embodiments, tanks 22and/or 24 have a rigid or semi-rigid construction. In some embodiments,the material comprising the walls of the tanks 22 and/or 24 areconfigured to flex outwardly when filled with electrolyte in order tocontain the electrolyte therein. As such, the tanks in some embodimentscan expand or contract to accommodate the expected range of changes inelectrolyte volume or pressure during operation.

In some embodiments, the anolyte tank 22 and/or catholyte tank 24 areconstructed such that some portions of the tanks are more rigid tosupport equipment or other features attached to the tanks, while otherportions of the tanks may retain flexibility as described above. Inaddition, the anolyte tank 22 and/or catholyte tank 24 may beconstructed such that some portions of the tanks are thicker forstructural support, while other portions of the tanks may be thinner toenable heat transfer, as described in greater detail below.

The catholyte tank 24 is configured to be substantially similar to theanolyte tank 22. In one embodiment of the present disclosure, thecatholyte tank 24 has a smaller volume than the anolyte tank 22, asdescribed in greater detail below. An optimized tank size ratio betweenthe anolyte and catholyte tanks 22 and 24 provides a means to maintainmaximum energy storage capacity of the RFB module 20 over multiplecycles. The difference in volume between the anolyte and catholyte tanks22 and 24 can be realized via the tank dimensions, for example, of theanolyte tank 22, or the tanks can have nearly identical footprints (seeFIG. 8) but the catholyte tank 24 may include a stepped section 90 (seeFIG. 7). In lieu of a stepped section, the smaller tank may include acavity bottom that is higher than the floor of the tank or a fillermaterial, such as an inert material, that takes up some of the volume ofthe tank. In other embodiments (not shown), the anolyte tank may havesubstantially the same volume as the catholyte tank or may have asmaller volume than the catholyte tank.

In some embodiments of the present disclosure, the anolyte tank 22 andthe catholyte tank 24 are configured so as to store a combined volume ofelectrolyte of about 1.0 cubic meters or greater. In one representativeembodiment, the total combined volume may be in the range of about 1.0to about 2.0 cubic meters.

As shown in FIG. 7, the tanks 22 and 24 are sized to fit closely intothe shell 50. For example, the length of each tank 22 and 24 is suchthat they abut against the walls for the main compartment 62 of theshell 50. Therefore, the walls of the main compartment 62 of the shell50 provide support to the tanks 22 and 24 to prevent deformation of thetanks 22 and 24. In one embodiment, the tanks may be configured to abutone another and provide support to each other along the abutting sides72 and 74 of the respective tanks 22 and 24(see FIG. 8). In anotherembodiment, the main compartment 62 may be configured with a center wallto support the tanks 22 and 24 to prevent deformation of the tanks 22and 24.

To increase rigidity and strength of the shell 50, and to withstandadditional side loading imparted by the electrolyte in the tanks 22 and24, the walls of the main compartment 62 of the shell 50 can bereinforced. In one embodiment, the shell 50 has a unibody construction.In another embodiment, the shell 50 includes multiple components. Thestructural support provided by the shell 50 when the tanks 22 and 24 arefilled with electrolyte allows for the tanks 22 and 24 to bemanufactured similarly to bladders that have minimal inherentself-supporting structure.

To reduce the possibility of an electrolyte leakage from the tanks 22and 24, the tanks 22 and 24 generally do not have penetrations below thelevel of the liquid stored in the tanks 22 and 24. In the illustratedembodiment, there is one penetration into each tank 22 and 24 slightlybelow the liquid level in one of the tanks to accommodate theelectrolyte transfer conduit 92 (see FIG. 3). As described in greaterdetail below, the electrolyte transfer conduit 92 is positioned near thetop of each tank 22 and 24 and allows for electrolyte flow between tanks22 and 24 to rebalance the volume of electrolyte in the tanks 22 and 24.

In the event of a leak of electrolyte in the RFB module 20, the maincompartment 62 of the shell 50 is manufactured to provide secondaryelectrolyte containment. As discussed above, the main compartment 62 ofthe shell 50 may be manufactured from a suitable material, and all seamsare fully welded or sealed to provide secondary leak containment. In theillustrated embodiment, the main compartment 62 of the shell 50 isdesigned as a tub.

Tank Heat Transfer and Tank Channel System

As the battery system 20 runs, heat may be generated and stored in theelectrolyte, and unwanted chemical vapors may build up in the shell 50.Referring to FIGS. 7 and 8, a tank heat transfer system is used forcontinuous air flow between an outer surface of the tank assembly 26 andan inner surface of the main compartment 62 of the shell 50. In theillustrated embodiment, some of the vertical exterior wall surfaces ofthe tanks 22 and 24 are designed and configured to enhance heat transferfrom the electrolyte to the external environment.

In the illustrated embodiment, exterior surfaces of the tanks 22 and 24include a plurality of abutments 82 for abutting the inner surface ofthe main compartment 62 of the shell 50, with airflow channels 84extending between adjacent abutments 82 for airflow between theabutments 82.

As seen in FIGS. 7 and 8, the airflow channels 84 are elongate channelsextending along the exterior surfaces of the tanks 22 and 24 when thetanks are abutted against each other in the shell 50. The airflowchannels 84 have inlets and outlets connected to first and secondmanifolds 86 and 88 to dissipate heat from the system. A fan 32 locatedin the first manifold 86 directs airflow through duct 34. In theillustrated embodiment, the fan 32 is pulling air through the channels,as indicated by airflow arrows A1. However, the fan 32 may be configuredfor reverse flow, or the fan 32 may be positioned in the second manifold88. Although shown in the illustrated embodiment as two manifolds 86 and88, there may be any number of manifolds in the system to optimize airflow around the tank assembly 26.

In accordance with embodiments of the present disclosure, the channels84 may be configured like an arch bridge to maximize the strength andheat dissipation effects of the walls of the tanks 22 and 24. As seen inFIG. 7, the abutments 82 of the tanks provide structural support forabutting the tanks 22 and 24 against inner surface of the maincompartment 62 of the shell 50. The channels 84 extending betweenadjacent abutments 82 are designed as arches to optimize the archaction, transferring the forces on the arches to the abutments 82. Suchdesign allows the tank wall strength to contain the volume ofelectrolyte, while also providing channels 84 for airflow passage alongthe exterior surfaces of the walls of the tanks 22 and 24.

In the illustrated embodiment, twelve airflow channels 84 are providedin the external vertical wall surfaces of the tank assembly 26. However,the number and sizing of the airflow channels 84 and the abutments 82may be designed and configured in accordance with system constraints fortank wall strength and desired heat dissipation. For example, systems 20residing in hotter or cooler climates may need more or less heatdissipation and/or more or less strength.

In accordance with embodiments of the present disclosure, the tanks 22and 24 may be manufactured with abutments 82 and airflow channels 84 tofurther optimize the structural and heat transfer properties of thetanks. Referring to FIGS. 9A and 9B, cross-sectional views of tank wallsare provided. In FIG. 9A, in accordance with one embodiment of thepresent disclosure, the walls of the tanks 22 and 24 have a uniformcross-sectional thickness across the abutments 82 and airflow channels84.

In FIG. 9B, in accordance with another embodiment of the presentdisclosure, walls of the tanks 22 and 24 have a non-uniformcross-sectional thickness across the abutments 82 and airflow channels84. In FIG. 9B, the abutments 82 are designed with a thickness T1 tooptimize tank strength, while the airflow channels 84 are designed witha variable thickness from T1 to T2 to optimize heat transfer across thetank wall in the airflow channels 84.

In one embodiment of the present disclosure, the tanks 22 and 24 aremanufactured from plastics via conventional molding processes, such asblow molding or injection molding, to achieve uniform wall thickness. Inanother embodiment, the tanks 22 and 24 are manufactured from plasticsvia other molding processes, such as rotational molding, to achievenon-uniform wall thickness.

In the illustrated embodiment, the tank assembly 26 is designed withchannels 84 for heat transfer along at least a portion of the exteriorvertical side wall surfaces of the tanks 22 and 24 in contact with themain compartment 62 of the shell 50. Along the interface 28 between thetanks 22 and 24, there are no channels 84. Therefore, the tanks 22 and24 support each other or rest against a dividing wall along thisinterface 28. However, in other embodiments, the tanks 22 and 24 mayinclude channels 84 along the interface 28.

Other channels in the tanks 22 and 24 on top, bottom, and upper wallsurfaces are configured for manufacturing advantages and for leakcontainment in the system 20. For example, referring to FIG. 8, topchannels 110 carry any electrolyte spills from the electrochemical cell30 away from the top surfaces of the tanks 22 and 24 to side containmentwithin the main compartment 62 of the shell 50. Likewise, shelf channels112 on the stepped shelf section 90 of the catholyte tank 24 carry anyelectrolyte spills from the stepped shelf section 90 to side containmentwithin the main compartment 62 of the shell 50. Referring to FIG. 9,interface channels 114 extending through the interface 28 between thetanks 22 and 24 to the bottom surfaces of the tanks 22 and 24 receiveand contain any electrolyte spills making their way to the interface 28.

Electrolyte Circulation System

As described above regarding the general operation of a RFB 20, anelectrolyte circulating system 40 is provided for circulating theanolyte and the catholyte from respective tanks 22 and 24 into theelectrochemical cell 30 (see FIG. 3). Referring to FIGS. 10 and 11, theelectrolyte circulation system 40 will now be described in greaterdetail.

Referring to FIGS. 10 and 11, anolyte and catholyte is delivered fromthe tank ports 132 and 134 of the respective anolyte and catholyte tanks22 and 24 to the electrochemical cell 30 using pumps 122 and 124. Aftertraveling through cell feed lines 152 and 154 of the electrolytecirculating system 40 to the electrochemical cell 30, anolyte andcatholyte is returned via anolyte and catholyte return lines 142 and146, which discharge to the respective anolyte and catholyte tanks 22and 24.

Referring to FIGS. 7 and 10 and 11, the electrolyte flow path of theanolyte will be described in greater detail. From the anolyte tank 22,anolyte travels along a horizontal path along the stepped shelf 90 fromanolyte tank port 132 through anolyte feed lines 152 to elbow 136 to theanolyte pump 122. From the anolyte pump 122, anolyte is pumped throughvertical feed line 152 and filter 150, continuing through anolyte feedlines 152 to the anolyte inlet 148 in the electrochemical cell 30.

Still referring to FIGS. 7 and 10 and 11, the electrolyte flow path ofthe catholyte will now be described in greater detail. From thecatholyte tank 24, catholyte travels along a horizontal path along thestepped shelf 90 from catholyte tank port 134 through catholyte feedlines 154 to elbow 138 to the catholyte pump 124. From the catholytepump 124, catholyte is pumped through vertical feed line 154 and filter150, continuing through catholyte feed lines 154 to the catholyte inlet144 in the electrochemical cell 30.

Seals at the respective tank ports 132 and 134 of the anolyte andcatholyte tanks 22 and 24 seal the holes 132 and 134 leading into thetanks 22 and 24 (see FIG. 7).

In the anolyte and catholyte tanks 22 and 24, the return system from theanolyte and catholyte outlets 142 and 146 of the electrochemical cell 30tank immersed return lines 156 and 158 extending into the tanks 22 and24. Seals at the respective upper walls of the anolyte and catholytetanks 22 and 24 seal the holes 160 and 162 leading into the tanks 22 and24 (see holes 160 and 162 in FIG. 7).

As seen in FIGS. 10 and 11, at the discharge ends 164 and 166 of anolyteand catholyte tank immersed return lines 156 and 158, the exit pipingcan be configured in at an angle relative to the walls of the tanks 22and 24 to encourage electrolyte mixing in the tanks 22 and 24.

Because the electrochemical cell 30 is disposed above the tanks 22 and24, pumping action is required to supply electrolyte to the cell 30. Theelectrolyte circulating system 40 is designed such that when the pumpingaction is turned off, most of the electrolyte in the cell 30 will drainback to the respective anolyte and catholyte tanks 22 and 24. Smallholes in the immersed return line 156 and 158 (not shown) let gas in topermit the stack and piping to drain.

Pump Assembly Configuration

The pump assembly 120 of the present disclosure provides electrolytecirculation via pumps 122 and 124 between the electrolyte storage tanks22 and 24 and the electrochemical cell 30 of the redox flow battery 20.Referring to FIG. 3, the pump assembly 20 of the illustrated embodimentis along the stepped shelf 90 on the catholyte tank 24. The maincompartment 62 of the shell 50 is configured to fully enclose theanolyte and catholyte tanks 22 and 24 (including the stepped shelf 90),as seen in FIGS. 7 and 10. As will be described in detail below, thepump assembly 120 is designed and configured for rotation above thewalls of the main compartment 62 for the pumps 122 and 124 and filters150 to be accessible for maintenance and replacement.

Referring to FIGS. 12A and 12B, the rotational design of the pumpassembly 120 will be described in greater detail. Referring to FIG. 12A,the pumps 122 and 124 of the pump assembly 120 nest on the stepped shelf90 and are supported by pump supports 126 and 128. To rotate the pumpsto the configuration shown in FIG. 12B, filters 150 are disconnectedfrom the respective electrolyte feed lines 152 and 154 (see FIGS. 10 and11) and the pumps 122 and 124 and filters 150 are rotated (see arrow A2)about the pivot axes of the anolyte and catholyte tank ports 132 and 134to be positioned above the top wall of main compartment 62.

To reengage the system, the pumps 122 and 124 and filters 150 can berotated back to their original position about the pivot axes of theanolyte and catholyte tank ports 132 and 134 and the filters 150 can bereconnected to the respective electrolyte feed lines 152 and 154 (seeFIGS. 10 and 11).

Because the pumps 122 and 124 are positioned on the stepped shelf 90 ina staggered configuration, both pumps can be rotated to theirpositioning above the top wall of main compartment 62 at the same time.

In accordance with embodiments of the present disclosure, the pumpassembly 120 is moveable between a first position in the maincompartment 62 of the shell 50 and below the electrolyte liquid heightduring operation of the pump assembly 120 and a second position abovethe electrolyte liquid height when the pump assembly 120 is notoperating. In the second position, the pump assembly 120 may be above orbelow the height of the main compartment 62 of the shell 50.

Referring now to FIGS. 13A-13D, coupling interface 130 of theelectrolyte feed lines 152 and 154 to the anolyte and catholyte tankports 132 and 134 allow for rotation. The coupling interface providesdouble radial o-ring seals that prevent leakage and allow for rotationwhile maintaining a seal.

Electrolyte Adjustments for Managing Energy Storage Capacity

As described previously, the relationship between electrolyteconcentration in the anolyte and catholyte tanks generally remainsconstant after the initial start-up phase. As the battery cycles, thevolume and active materials in the anolyte and catholyte tanks canchange as a result of inherent chemical reactions, the nature of thebattery cell structure, and other factors. Without electrolyterebalancing between the anolyte and catholyte tanks, the battery energycapacity degrades over time as the result of limited availability ofactive material in the anolyte or catholyte tank. Therefore, a systemthat maintains a specific electrolyte concentration ratio between theanolyte and catholyte tanks and/or maximizes the available activematerials for energy storage and dispatch is described herein.

Electrolyte Volume Ratio

In one embodiment of the present disclosure, a method of operating aredox flow battery includes having a uniform or non-uniformpredetermined volume ratio, based on maintaining a preferred electrolyteconcentration, between the quantity of anolyte and the quantity ofcatholyte in the system. In the case of non-uniform predetermined volumeratio, the quantity or volume of anolyte may be more or less than thequantity or volume of the catholyte. The predetermined starting volumeratio may be different from or the same as the predetermined volumeratio during operation. Moreover, the predetermined volume ratio duringoperation may change subject to other conditions in the system.

As non-limiting examples, the tank volume ratio may have an anolytevolume to catholyte volume ratio of about 1:1.05 to about 1:1.50, about1:1.15 to about 1:1.35, or about 1:1.20 to about 1:1.30. As anon-limiting example, in the illustrated embodiment of FIG. 2, the tankvolume ratio between the anolyte tank and the catholyte tank is about1.25:1.

As other non-limiting examples, the tank volume ratio may have acatholyte volume to anolyte volume ratio of about 1:1.05 to about1:1.50, about 1:1.15 to about 1:1.35, or about 1:1.20 to about 1:1.30.

A non-uniform tank volume ratio may be achieved by having two differenttank designs. For example, see the tank configurations in theillustrated embodiment of FIGS. 7 and 10. As described above, theanolyte and catholyte tanks 22 and 24 have similar footprint dimensions(see FIG. 8), but the catholyte tank 24 includes a stepped shelf section90 near (see FIG. 7). In other embodiments, the tanks may have differentfootprint dimensions or different height dimensions. In someembodiments, the volume of electrolyte in the tanks may be different,but the anolyte and catholyte tanks 22 and 24 have substantially thesame liquid height level to allow for an overflow conduit 92 to maintaina specified tank volume ratio. In other embodiments, the tanks may bepartially filled with non-reacting materials to reduce some of the tankvolume, or the tank may have a changeable volume to account for changesin the operation of the system.

As described above, a non-uniform tank volume ratio based on maintaininga preferred electrolyte concentration between the anolyte and catholytetanks can improve the energy density achieved during operation of theRFB module 20 having a given capacity for holding a certain amount ofelectrolyte. As a non-limiting example, a tank volume ratio of apreferred, non-uniform, electrolyte concentration, such as about 1.25:1,between the anolyte tank and the catholyte tank in the illustratedembodiment of FIGS. 7 and 10 may achieve and maintain greater energydensity for the same total amount of electrolyte as compared to auniform tank volume ratio between the anolyte and catholyte tanks.Greater energy density is a result of greater availability andutilization of the active species in the electrolyte. In other types ofmodules, for example, in non-vanadium RFB systems, a preferable tankvolume ratio may vary from the preferred range for a VRB system, and forexample, may have a greater volume of catholyte compared to anolyte.

Electrolyte Transfer

In accordance with one embodiment of the present disclosure, the RFB 20has a predetermined volume ratio, based on maintaining a preferredelectrolyte concentration, in accordance with the volume ratios ofanolyte and catholyte, as described above. Over a period of time ofnormal operation of the redox flow battery, the volume ratio of theanolyte and the catholyte may become greater than or less than thepredetermined volume ratio. For example, in one mode of operation, a VRBsystem gains catholyte volume and loses anolyte volume over long-termcycling.

Therefore, in accordance with embodiments of the present disclosure, avolume of catholyte from the catholyte storage tank 24 to the anolytestorage tank 22, or a volume of anolyte from the anolyte storage tank 22to the catholyte storage tank 24, to restore the volume ratio to thepredetermined volume ratio. In the exemplary schematic of FIG. 14,excess catholyte generated from the system would flow from the catholytetank 24 to the anolyte tank 22 to correct the volume imbalance.

Such transfer may be affected by passive electrolyte transfer, activeelectrolyte transfer, or a combination of passive and active electrolytetransfer, all described in greater detail below.

In one embodiment of the present disclosure, a passive mechanicalarrangement allows for the transfer of electrolyte between the anolyteand catholyte tanks. The transfer may be from anolyte tank 22 tocatholyte tank 24 or from catholyte tank 24 to anolyte tank 22.

As seen in FIG. 3, the passive transfer system is a tank electrolytetransfer conduit 92. Referring to the simplified schematic in FIG. 14,the electrolyte transfer conduit 92 is located at an overflow level ineither the catholyte or anolyte tank 22 or 24. As discussed above, astepped shelf section 90 in the catholyte tank 24 allows the electrolytetransfer conduit 92 to nest within the main compartment 62 of the shelland to provide fluid communication between the anolyte tank 22 and thecatholyte tank 24 when the tanks are aligned side-by-side.

In this configuration, the flow rate of electrolyte between the tanks 22and 24 is determined based on the electrolyte level differences in thetanks 22 and 24. As seen in the illustrated embodiment of FIG. 7, thecatholyte tank 24 is sized to have a smaller volume than the anolytetank 22 by having a stepped shelf portion 90. The electrolyte transferconduit 92 that extends between conduit connection points 170 and 172(see also FIG. 15) is located at the overflow level allows for the flowof catholyte from the catholyte tank 24 as the catholyte volumeincreases into the anolyte tank 22 (or vice versa).

The electrolyte transfer conduit 92 is designed to penetrate each tank22 and 24 at or slightly below the liquid level to accommodateelectrolyte transfer conduit 92. To prevent any leaks that may occur atthe joints between the conduit 92 and the tanks 22 and 24, the conduitconnection holes 170 and 172 with each tank 22 and 24 may include leakprevention devices, such as unions, axial O-ring fittings, etc.

In embodiments of the present disclosure, the electrolyte transferconduit 92 in each of the tanks 22 and 24 may be set so as to allow forthe transfer of only liquid electrolyte or of both liquid electrolyteand gas (from the headspaces 66 and 68 in the anolyte and catholytetanks 22 and 24) through the electrolyte transfer conduit 92. If atransfer of gas from the headspaces 66 and 68 in the anolyte andcatholyte tanks 22 and 24 is provided, the electrolyte transfer conduit92 is also a part of the gas management system 94 for the battery, asdescribed in greater detail below.

As can be seen in FIG. 12A, the electrolyte transfer conduit 92 of theillustrated embodiment is configured with a slightly lower elevationchange between the connection points to the anolyte and catholyte tanks22 and 24. The elevation change permits only fluid transfer. In theillustrated embodiment, a separate gas crossover 98 provides for freegas exchange between the anolyte and catholyte headspaces 66 and 68 (seeFIGS. 14 and 15).

In one embodiment of the present disclosure, the electrolyte transferconduit 92 delivers excess catholyte to the anolyte tank 22 duringoperation to account for the volumetric increase in the catholyte andreturn the system to a predetermined volume ratio.

In accordance with other embodiments of the present disclosure, thetanks 22 and 24 need not be manufactured to include a stepped shelfsection 90 or may include another configuration to accommodate either anelectrolyte transfer conduit or another fluid transfer device betweentanks 22 and 24. For example, a suitable electrolyte transfer conduitmay be located not at an overflow position, but instead below the liquidlevel in each of the tanks. In such a configuration, the electrolytetransfer conduit would provide continuous electrolyte exchange betweenthe anolyte and catholyte. The rate of exchange may be determined inpart by the length and diameter of the transfer conduit.

In addition to, or in lieu of the passive transfer system such aselectrolyte transfer conduit 92, the RFB module 20 may include an activetransfer system configured for actively transferring electrolyte fromone to the other of the anolyte and catholyte tanks. Such activetransfer may include pumping or otherwise controlling electrolytetank-to-tank transfer using a valve system. The active transfer may beautomatically controlled based on system conditions or manuallycontrolled by an operator.

If a combination of passive and active electrolyte transfer systems isemployed, the active system may use the same or a separate electrolytetransfer conduit as the passive system.

Gas Generation During Operation

Most RFBs have side reactions, such as hydrogen generation. Hydrogengeneration increases the average oxidation state of the electrolytes,which can result in a capacity decrease. In addition, hydrogen gasgeneration in a closed space can create fire and safety concerns.Further, most RFB negative electrolyte solutions include strongreductants that can be oxidized by oxygen in the air. Such oxidation canincrease the average oxidation state of the electrolytes, which canresult in a capacity decrease.

For chloride-containing redox flow battery systems, a small amount ofchlorine gas may be generated, which can also create fire and safetyconcerns. Chlorine gas is a strong oxidant, and therefore, can beabsorbed by the negative electrolyte solutions through surface contactif the chlorine gas is permitted to travel to the headspace 66 of theanolyte tank 22, as discussed below with reference to a gas managementsystem.

Gas Management System

A gas management system 94 can be employed to manage the gassesgenerated in a redox flow battery 20. Although the gas management systemdescribed herein is designed for a vanadium redox flow battery, the samegas management system concepts may be applied to other non-vanadiumredox flow batteries.

With reference to the simplified schematic in FIG. 14, the components ofthe gas management system 94 will now be described. As discussed above,anolyte and catholyte tanks 22 and 24 are in a substantially sealedsystem with liquid electrolyte in each tank, and each tank may include aheadspace 66 and 68 above the respective anolyte and catholyte. In theillustrated embodiment, the headspaces 66 and 68 above the anolyte andcatholyte may have free gas exchange with the respective anolyte andcatholyte via gas exchange conduit 98 (see also FIG. 15).

In the illustrated embodiment, the gas management system 94 includes thegas headspaces 66 and 68, a gas transfer device 98 between the anolyteand catholyte tanks 22 and 24 one or more other gas transfer devices140, and a gas pressure management system 96 (shown as U-tube 100, to bedescribed in greater detail below).

During operation, anolyte and catholyte tanks 22 and 24 are filled withelectrolyte up to a fill line allowing for a headspace 66 and 68 in eachtank 22 and 24 (see FIG. 11A), and then sealed. The RFB system 20 isstarted in operation and the gas compositions of the headspaces start tochange as oxidation starts to occur and hydrogen starts to be generated.In one mode of operation, air is present in the respective headspaces ofthe anolyte and catholyte headspaces during electrolyte filling or othermaintenance operations when the tanks are sealed. In another embodiment,the headspaces are purged with nitrogen or another inert gas as part ofthe sealing process.

As discussed above, the tank system may include a tank electrolytetransfer conduit 92 located at or below an overflow level in either theanolyte or catholyte tank 22 or 24. In some embodiments, the electrolytetransfer conduit 92 may allow for the transfer of liquid electrolyte andgas exchange from the headspaces in the anolyte and catholyte tanks 22and 24.

In other embodiments, the gas transfer device 98 may be an independentgas transfer device different from the electrolyte transfer conduit 92.For example, the gas transfer device may be a conduit designed for gasexchange between the anolyte and catholyte headspaces 66 and 68, but notfor liquid electrolyte transfer.

The gas transfer system 94 provides a means to equalize the pressurebetween the anolyte and catholyte tanks, control the flow and exitlocation of gasses vented by the gas management system, and allows fordiffusion of gas between the anolyte and catholyte tanks.

In one embodiment of the present disclosure, for example, a VRB,chlorine gas generated in the catholyte tank 24 by the followingequation diffuses through the gas transfer device 98 and moves to theheadspace in the anolyte tank 22.Cl₂+2V²⁺=>2V³⁺+2Cl⁻

When in the anolyte headspace over the anolyte surface, the chlorine gasis absorbed by the anolyte as it is reduced to Cl⁻. The chlorine gasreduces quickly in the anolyte tank, before it has a chance to vent fromthe gas management system 94 through the gas pressure management system96, described below.

As seen in FIGS. 7, 11, and 15 in the illustrated embodiment, a gastransfer system 140 between the catholyte and anolyte headspaces will bedescribed. The gas from the catholyte headspace 68 is vented throughhole 176 (see FIG. 7) to a venturi 180 that creates a vacuum and drawsgas from the catholyte headspace 68 into the anolyte return line 142from the electrochemical cell 30. By combining into the return line 142,the treated gas from the catholyte headspace 68 makes its way into theanolyte tank 22 at or below the liquid level of the anolyte.

On its path from the catholyte headspace 68 to the anolyte return line142, the gas passes through a UV treatment zone 178 to be pre-treatedwith UV light. Such UV light treatment is a catalyst which causes thechlorine and hydrogen gases to form hydrogen chloride vapor. Such HClvapor is more readily absorbed by the anolyte, reducing the risk ofchlorine gas accumulation in the gas crossover lines or chlorine gasleaks.

The formation of HCl from Cl₂ and H₂ is an exothermic reaction, whichproduces heat. If the chlorine levels venting from the catholyteheadspace 68 to the gas transfer system 140 and the UV treatment zone178 are high, significant heat will be produced by the exothermicreaction. Therefore, an optional heat sensor (not shown) in or near thegas transfer system 140 can be an indicator of abnormal chlorine levelsin the system, which can provide information to the control to adjustthe system parameters, for example, to stop charging or to dischargeenergy from the system.

Gas Pressure Control

In addition to one or more gas transfer devices, the gas managementsystem 94 may also include one or more gas pressure management systems96 to maintain a barrier between ambient air and the gas managementsystem 94, control gas pressure in the headspaces 66 and 68 of the gasmanagement system 94, and allow any necessary bi-directional pressureequalization between ambient air and the gas management system 94. Inthat regard, the gas pressure management device 96 may allow for therelease of excess hydrogen gas generated by the anolyte in the anolytetank 22. The gas pressure management device 96 may also release carbondioxide and nitrogen, and any other gases that may build up in the gasmanagement system 94. However, as discussed above, any chlorine gasgenerated by a system (such as a vanadium redox flow battery containingchloride) tends to be absorbed by the anolyte if the chlorine gas isallowed to migrate from the headspace 68 in the catholyte tank 24 to theheadspace 66 in the anolyte tank 22 through gas transfer device 92.

Referring to FIG. 15, in accordance with one embodiment of the presentdisclosure, the gas pressure control device is a U-shaped tube (U-tube)100 in fluid communication with the headspace 66 of the anolyte tank 22(see anolyte vent hole 174 to U-tube in FIG. 8). Although shown in fluidcommunication with the headspace 66 of the anolyte tank 22, the U-tube100 could also be suitable configured to be in fluid communication withthe headspace 68 of the catholyte tank 24.

As illustrated in FIG. 8, a hole 174 provides an access position for theU-tube 100 to the head space 68 in the anolyte tank 22. However, othersuitable connector points are within the scope of the presentdisclosure. In the illustrated embodiment, the U-tube 100 is positionedto reside in the space created by the stepped shelf 90 on the catholytetank 24.

In the illustrated embodiment of FIG. 15, the U-tube 100 has a U-shapedbody 102 and a first end 104 in fluid communication with the headspace66 of the gas management system 94 and a second open end 106 in fluidcommunication with an external atmosphere. The body 102 contains anamount of liquid 108 (see FIG. 14) that remains in the plumbing trapcreated by the U-shaped body 102 between the first and second ends 104and 106.

In the illustrated embodiment, the U-tube body 102 is a conduit whichmay have a constant cross-sectional area along the length of the U-tubefrom the first end 104 to the second end 106. In another embodiment, theU-tube body 102 may have a different cross-sectional area at the firstend, as compared to the second end.

The U-tube body may be designed to include baffles or enlarged sectionsto prevent the loss of liquid as a result of bubbling or a suddendischarge of gas.

As non-limiting examples, the U-tube may be filled with a liquidselected from the group consisting of water, an alkaline aqueoussolution, propylene glycol, ethylene glycol, an aqueous solution ofinorganic compound, an aqueous solution of organic compound, a waterinsoluble organic liquid, and combinations thereof, through whichcertain gases in the headspaces of the RFB will diffuse. A suitableliquid may be selected depending on the system, operating pressures, andtypes of gasses being emitted from the gas management system 94. Othersuitable liquids may be selected to provide certain operatingcharacteristics, for example, a desired temperature range or an abilityto scrub or eliminate undesired vent gases (such as chlorine) fromatmospheric discharge. In some non-limiting examples, the U-tube 100 mayinclude a combination of liquids, for example, an alkaline solution withan oil layer on top.

The U-tube 100 may also be filled with a buffer solution for absorbingacidic or acid-forming gases, such as HCl and chlorine gas. The buffersolution can include a pH indicator to show it has become acidified andneeds to be changed.

The U-tube 100 of the present disclosure allows for bi-directional gasexchange between the gas management system and the atmosphere. In theillustrated embodiment, the U-tube 100 is in fluid communication withthe anolyte headspace 66 in the anolyte tank 22 and the atmosphere. Inone non-limiting example, the U-tube 100 may include, for example, 10inches of water. In this example, when the pressure inside the anolyteheadspace exceeds 10 inches of water, gases such as hydrogen may startto bubble out of the tube into the atmosphere.

The U-tubs 100 may be configured to allow entry of an external gas intothe gas management system when an exterior battery pressure exceeds aninterior battery pressure, for example, greater than or equal to 15inches water. In the same example, the U-tube will prevent the entry ofan external gas into the anolyte storage tank when the exterior batterypressure exceeds the interior battery pressure by less than 15 incheswater. In addition, the tank head space may have some flexibility toallow for expansion.

In one embodiment, the U-tube 100 may have a uniform cross-section atthe first and second ends. In another embodiment, a U-tube 200 may havea different cross-sectional area at the first end, as compared to thesecond end. The effect of a change in cross-sectional area is that thepressure set points for gas entering and leaving the gas managementsystem may be different. For example, the first and second endcross-sectional areas may be sized so that the pressure requirement forgas exiting the gas management system is 15 inches of water, but thepressure requirement for gas entering the gas management system from theatmosphere is only 6 inches of water. In one embodiment of the presentdisclosure, the interior battery pressure in the anolyte headspace isbetween −10 kPa and 10 kPa, −5 kPa to +5 kPa, and −3 kPa to +3 kPa.

In accordance with other embodiments of the present disclosure, the gaspressure management device may include more than one U-tube device, oneor more pressure regulating valves, one or more check vales, or acombination of these or other pressure management devices.

As discussed above, hydrogen generation can be a concern in RFBs. Inthat regard, hydrogen in combination with other gases may reach aflammability limit and pose a risk of ignition. The closed gasmanagement system mitigates this risk by keeping constituent gases intank head spaces below flammability limits.

Open-Circuit Voltage Cell

Referring to FIG. 6, an open-circuit voltage (OCV) cell 116 can be usedto measure the state of charge (SOC) of the redox flow battery 20. In aredox flow battery, it is also generally desirable for the state ofcharge (SOC) of the anolyte and the catholyte to be matching or close tomatching. Matching SOC between the anolyte and catholyte can helpmitigate unwanted side reactions in the system (which may generateunwanted hydrogen if the anolyte SOC is too high or unwanted chlorine ifthe catholyte SOC is too high, if chloride species containingelectrolytes are used in the battery). When the SOC values of theanolyte and catholyte are known, the system can be adjusted to return tothe target values or target value ranges.

For “matching”, the acceptability of the difference between the SOCvalues of the anolyte and the catholyte depends on the battery system.In one embodiment of the present disclosure, the difference between theSOC values of the anolyte and the catholyte is less than 20%. In oneembodiment of the present disclosure, the difference between the SOCvalues of the anolyte and the catholyte is less than 10%. In anotherembodiment of the present disclosure, the difference between the SOCvalues of the anolyte and the catholyte is less than 5%. In anotherembodiment of the present disclosure, the different between the SOCvalues of the anolyte and the catholyte is reduced to mitigate sidereactions to an acceptable level.

The SOC values of the anolyte and the catholyte can change over timewith multiple cycles, often becoming unbalanced or unmatched over time.During operation, real-time monitoring of the status of the electrolytesin a RFB provides information on the operation of the RFB. Real-timemonitoring of SOC is typically achieved by measuring the OCV of thepositive and negative electrodes using a single-cell type OCVmeasurement device (see FIG. 6). (Other ways of determining SOC besidesOCV are also within the scope of the present disclosure, such asrecording and analyzing the amount of energy entering and leaving thebattery over a given time period, which may be referred to as coulombcounting.)

Battery Energy Density

Evolving demands and applications for large-scale energy storage systemsdrive the requirement for energy dense packaging that provides siteflexibility and ease of installation. Many RFB systems have relativelylow system level energy density, due in part to the combination of theirmethods of system packaging, for example the use of traditional externaltanks, or multiple containers that house the tanks separately from thebalance of plant (BOP). Other limitations of traditional system energydensity may be due to the inherent chemistry of the electrolyte, limitedspace availability for subsystems that manage shunt current losses,gasses, electrolyte utilization, or a combination of factors.

In accordance with aspects of the present disclosure, the tanks, thecontainer, and the remaining balance of plant system, such as thosedescribed above, can be configured as a self-contained, substantiallyclosed VRB unit that provides maximum energy storage capacity per unitsize of the container, while maintaining safe and reliable operatingcriteria. As such, RFB module 20 shown in FIGS. 1 and 2 constructed inaccordance with embodiments of the present disclosure can be configuredto have an energy density of 20 watt hours per liter of electrolyte(Wh/L) or greater for an RFB battery that has an energy capacity of atleast 2 kW-hours.

The RFB module 20 in embodiments of the present disclosure also may bedesigned to operate continuously while maintaining designed energydensity for a minimum of 50 or a minimum of 100 continuous fullcharge/discharge cycles or the equivalent operating hours withoutinterruption by service or user input.

General Arrangement

As discussed above the RFB module 20 described herein, as can be seen inFIGS. 1 and 2, is designed to be contained in a shell 50 having specificdimensions. having dimensions designed to fit between a standardcommercial single doorway (32″×80″). Space usage for the variouscomponents in the system can be optimized to maximize the amount ofelectrolyte that can be filled into the shell 50. As will be describedin more detail below, configuration of the battery, battery sub-systems,or components themselves as well as the synergistic combinations ofthese elements allow the RFB 20 to achieve the specified energy density,both initially and continuously over a period of time.

Referring to FIGS. 1 and 2, a forklift interface 38 can be provided onthe bottom of in the shell 50 for ease of shipping. The forkliftinterface 38 can be designed to be able to be picked up and maneuveredby a standard capacity pallet jack from the front side of the battery20. The shell 50 can be designed to fit between a standard commercialsingle doorway (32″×80″).

Operational Features

In addition to space utilization features, one or more operationalpassive or active management features can be employed to improve theoperational efficiency of the RFB module and to also extend thecontinuous operational period of the RFB module without shutdown.

As noted above, in addition to maximizing the amount of electrolytecontained in the system to maximize energy density, the RFB system isalso designed to maintain such energy density over a certain number ofcycles, for example, 100 full charge/discharge cycles. To help maintainsystem capacity, one or more adjustments can be made to the electrolyteduring operation of the battery. For example, as the anolyte andcatholyte volumes deviate from a predetermined volume, the system can bedesigned for a constant or periodic transfer of electrolyte from thecatholyte to the anolyte (or anolyte to catholyte) to maintainpredetermined tank electrolyte volumes, whether by active or passiveelectrolyte transfer methods. Moreover, individual batteries canautomatically be periodically adjusted to conform to a selected OCVvalue in a string to improve long-term performance.

In addition, an optional gas management system can be employed to removeor minimize reactions that decrease performance over time and mitigatethe effects of evolved gases from the electrolyte. Such gases, if leftunchecked, could be harmful to the system, create a safety hazard, orrequire environmental emissions monitoring, particularly chlorine andexcess hydrogen gas that may be generated in a RFB.

Electrolyte Composition

In addition to space management for maximizing the amount of electrolytecontained in the system to maximize energy density, the electrolyteitself may be formulated to enhance the energy storage capacity of theRFB. In accordance with embodiments of the present disclosure, in avanadium redox flow battery, vanadium concentration is selected from thegroup consisting of higher than 1.5M, higher than 1.8M, and higher than2.0M.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the present disclosure.

The embodiments of the present disclosure in which an exclusive propertyor privilege is claimed are defined as follow:
 1. A redox flow battery(RFB), comprising: a shell; an electrolyte storage tank assemblydisposed in the shell, wherein at least a portion of the electrolytestorage tank assembly is supported by the shell and wherein at least aportion of the electrolyte storage tank assembly defines a tank assemblyheat transfer system between an outer surface of the electrolyte storagetank assembly and an inner surface of the shell, wherein the tankassembly heat transfer system includes a plurality of horizontal airflowchannels defined in an exterior surface of the electrolyte storage tank,wherein the heat transfer system is defined at least in part by theairflow channels between the exterior surface of the electrolyte storagetank and the inner surface of the shell, wherein the exterior surface ofthe electrolyte storage tank has a plurality of abutment regions betweenthe plurality of horizontal airflow channels wherein the plurality ofabutment regions is configured to make contact with and be supported bythe inner surface of the shell, wherein the thickness of the exteriorsurface of the electrolyte storage tank is variable along the height ofeach of the plurality of horizontal airflow channels such that thethickness is greatest at the plurality of abutment regions and thethickness is reduced between the abutment regions; an electrochemicalcell; and an electrolyte circulation system configured for fluidcommunication between the electrolyte storage tank assembly and theelectrochemical cell.
 2. The redox flow battery (RFB) of claim 1,wherein the heat transfer system includes the plurality of horizontalairflow channels and an air circulation device.
 3. The redox flowbattery (RFB) of claim 1, wherein the tank assembly heat transfer systemincludes a plurality of tank abutment regions and a plurality ofhorizontal airflow channels, with two tank abutment regions adjacenteach channel.
 4. The redox flow battery (RFB) of claim 1, wherein theredox flow battery is a vanadium redox flow battery.
 5. The redox flowbattery (RFB) of claim 1, wherein the electrolyte storage tank assemblyincludes a catholyte tank and an anolyte tank.
 6. The redox flow battery(RFB) of claim 5, wherein the catholyte tank and the anolyte tank are ina side-by-side configuration in the shell.
 7. The redox flow battery(RFB) of claim 5, wherein each of the catholyte tank and the anolytetank define a portion of the tank assembly heat transfer system betweenan outer surface of each tank and an inner surface of the shell.
 8. Theredox flow battery (RFB) of claim 5, wherein the anolyte tank has avolume and wherein the catholyte tank has a volume, the ratio of thevolume of the anolyte tank to the volume of the catholyte tank being inthe range of 1.05:1 to about 1.5:1.
 9. The redox flow battery (RFB) ofclaim 8, wherein the catholyte tank and the anolyte tank havesubstantially the same footprint in contact with a bottom surface of theshell.
 10. The redox flow battery (RFB) of claim 8, wherein thecatholyte tank and the anolyte tank have substantially the same liquidlevel.
 11. The redox flow battery (RFB) of claim 8, wherein thecatholyte tank includes a stepped shelf to reduce the volume of thecatholyte tank compared to the anolyte tank.
 12. A tank and shellsecondary containment system, the system comprising: a shell; and a tankdisposed within the shell, wherein at least a portion of the tank issupported by the shell and wherein at least a portion of the tankdefines a heat transfer system between an outer surface of the tank andan inner surface of the shell, wherein the heat transfer system includesa plurality of air flow channels and an air circulation device, whereinthe heat transfer system includes a plurality of horizontal airflowchannels defined in an exterior surface of the tank, wherein the heattransfer system is defined at least in part by the airflow channelsbetween the exterior surface of the tank and the inner surface of theshell, wherein the exterior surface of the tank has a plurality ofabutment regions between the plurality of horizontal airflow channelswherein the plurality of abutment regions is configured to make contactwith and be supported by the inner surface of the shell, wherein thethickness of the exterior surface of the electrolyte storage tank isvariable along the height of each of the plurality of horizontal airflowchannels such that the thickness is greatest at the plurality ofabutment regions and the thickness is reduced between the abutmentregions.
 13. A method of heat transfer for a redox flow battery (RFB),the method comprising: operating a redox flow battery having a shell, anelectrolyte storage tank assembly disposed in the shell, wherein atleast a portion of the electrolyte storage tank assembly is supported bythe shell and wherein at least a portion of electrolyte storage tankassembly defines a tank assembly heat transfer system between an outersurface of the electrolyte storage tank assembly and an inner surface ofthe shell, an electrochemical cell, and an electrolyte circulationsystem configured for fluid communication between the electrolytestorage tank and the electrochemical cell; and circulating air throughthe tank assembly heat transfer system between an outer surface of theelectrolyte storage tank assembly and the inner surface of the shell,wherein the tank assembly heat transfer system includes a plurality ofhorizontal airflow channels defined in an exterior surface of theelectrolyte storage tank, wherein the heat transfer system is defined atleast in part by the airflow channels between the exterior surface ofthe electrolyte storage tank and the inner surface of the shell, whereinthe exterior surface of the electrolyte storage tank has a plurality ofabutment regions between the plurality of horizontal airflow channelswherein the plurality of abutment regions is configured to make contactwith and be supported by the inner surface of the shell, wherein thethickness of the exterior surface of the electrolyte storage tank isvariable along the height of each of the plurality of horizontal airflowchannels such that the thickness is greatest at the plurality ofabutment regions and the thickness is reduced between the abutmentregions.
 14. The redox flow battery (RFB) of claim 1, wherein theelectrolyte storage tank assembly is made from plastic, and the shell ismade from metal.
 15. The redox flow battery (RFB) of claim 6, wherein adividing wall is disposed between adjacent catholyte and anolyte tanks.16. The redox flow battery (RFB) of claim 1, wherein the shell has ashell height and wherein the electrolyte storage tank assembly has anelectrolyte liquid height which is at or below the shell height, theredox flow battery further comprising an electrolyte circulation systemconfigured for fluid communication between the electrolyte storage tankassembly and the electrochemical cell, wherein the electrolytecirculation system includes a pump assembly, wherein the pump assemblyis moveable between a first position in the shell and below theelectrolyte liquid height during operation of the pump assembly and asecond position and above the electrolyte liquid height when the pumpassembly is not operating.
 17. The redox flow battery (RFB) of claim 16,wherein the pump assembly is coupled to first and second connections ofthe electrolyte circulation system when in the first position.
 18. Theredox flow battery (RFB) of claim 17, wherein the pump assembly isuncoupled from a first connection to the electrolyte circulation systemand remains coupled to a second connection to the electrolytecirculation system when in the second position.
 19. The redox flowbattery (RFB) of claim 18, wherein the pump assembly is rotatablebetween the first position and the second position while coupled to thesecond connection in the electrolyte circulation system.
 20. The redoxflow battery (RFB) of claim 1, wherein the electrolyte storage tankassembly includes an anolyte storage tank configured for containing aquantity of an anolyte and an anolyte headspace, and a catholyte storagetank configured for containing a quantity of a catholyte and a catholyteheadspace, and further comprising a gas management system including afirst gas exchange device having a first end in fluid communication withthe catholyte headspace and a second end in fluid communication withanolyte in the anolyte storage tank.
 21. The redox flow battery (RFB) ofclaim 20, wherein the first gas exchange device includes a gas treatmentzone for treating evolving gas that is evolving from the catholyte. 22.The redox flow battery (RFB) of claim 20, wherein the evolving gasincludes oxygen, carbon dioxide, hydrogen, and chlorine, and anycombination thereof.
 23. The redox flow battery (RFB) of claim 21,wherein the gas treatment zone includes UV treatment.
 24. The redox flowbattery (RFB) of claim 23, wherein the UV treatment promotes therecombination of hydrogen and chlorine gas into hydrogen chloride. 25.The redox flow battery (RFB) of claim 20, wherein the first gas exchangedevice includes a vacuum to draw gas from the catholyte headspace. 26.The redox flow battery (RFB) of claim 20, wherein the first end of thefirst gas exchange device includes a venturi.
 27. The redox flow battery(RFB) of claim 20, wherein the second end of the first gas exchangedevice is at or below the liquid level in the anolyte.
 28. The redoxflow battery (RFB) of claim 20, wherein the gas management systemfurther includes a second gas exchange device for gas exchange betweenthe catholyte headspace and the anolyte headspace.
 29. The redox flowbattery (RFB) of claim 21, wherein the gas treatment zone includes aheat sensor.
 30. The redox flow battery (RFB) of claim 20, wherein thegas management system further includes a third gas exchange deviceconfigured to contain or release an evolving gas from either or both ofthe anolyte and catholyte storage tanks to an exterior batteryenvironment when an interior battery pressure exceeds an exteriorbattery pressure by a predetermined amount.
 31. The redox flow battery(RFB) of claim 30, wherein the third gas exchange device comprises anarrangement of one or more of a liquid-filled U-shaped tube, apressure-regulated valve, a pressure relief valve, or a check valve.